terms of employing the ideal bandgap absorber layer in PSC, the tin-lead (Sn-Pb) mixed PSCs are now getting attention with the additional benefit of utilizing them in tandem solar cell technology. [8][9][10] In recent years, various research groups have demonstrated power conversion efficiencies (PCEs) of more than 20% by employing Sn-Pb PSCs. [9,[11][12][13][14] However, PCE of Sn-Pb mixed PSCs is still lagging behind their Pb counterparts, especially in terms of open-circuit voltage (V oc ) loss, which is conventionally described as the deficit from the bandgap, is less than 0.3 V for the efficient Pb-PSCs. [4][5][6] Therefore, in the recent past, the efforts are directed toward finding the solutions to overcome the V oc loss. Researchers around the world are trying to address the problem by solving the issues related to the physical properties of the Sn-Pb perovskite films such as short carrier lifetime, [12] large Urbach energy, [11] high trap density, and most importantly the rapid oxidation of Sn 2+ to Sn 4+ . [9,13,14] The focus has been on improving the physical properties by employing different thin film formation strategies. Tong et al., [12] demonstrated the drastic improvement in optoelectronic properties such as the increase in carrier lifetime of more than 1 µs and carrier diffusion length of longer than 1 µm with the incorporation of bulky guanidinium thiocyanate (GuaSCN) into the perovskite films that led to the first research report of more than 20% PCE in Sn-Pb PSCs with a V oc loss of 0.42 V. The increase in PSC performance is assigned to the passivation by the formation of a 2D structure at the grain boundaries that also suppresses the formation of Sn vacancies. This kind of improvement in solar cell performance is the same observation as reported in the case of pure Pb-containing PSCs. [15] Our group also showed the effect of the decrease in trap densities, at the surface and bulk, by using strain engineering, steering to the PCE of 20.4%, and V oc loss of less than 0.50 V. [11] Recently, Li et al., [16] also demonstrated the importance of surface and grain boundary passivation by the formation of 1D pyrrolidine perovskite, a V oc loss of 0.41 V is reported. Lin et al., [9] addressed the oxidation problem in Sn-Pb precursor solution. The Sn metal is introduced as a reducing agent in precursor solution that decreased concentration of Sn 4+ (Sn 4+ + Sn→ 2Sn 2+ ) in the precursor solution before the film formation, Tin-lead perovskite solar cells (PSCs) show inferior power conversion efficiency (PCE) than their Pb counterparts mainly because of the higher open-circuit voltage (V oc ) loss. Here, it is revealed that the p-type surface of perovskite transforms to n-type, based on post-treatment by a Lewis base, ethylenediamine. This approach forms a graded band structure owing to the rise of the Fermi-energy level at the surface of the perovskite layer, and increases the built-in potential from 0.56 to 0.76 V, which increases the V oc by more than 100 mV. It is demonstrated that EDA can lower the ...